Close approaches and acoustic triangulation: techniques for mapping the distribution of booming Australasian bittern (Botaurus poiciloptilus) on small wetlands

Reliable techniques need to be developed for monitoring the distribution and abundance of the endangered Australasian bittern (matuku, Botaurus poiciloptilus). We trialled 2 complementary methods for estimating the number of booming Australasian bitterns: acoustic triangulation from fixed listening stations, and using kayaks to quietly approach booming birds. Trials were conducted over 2 booming seasons (2011 and 2012) at Hatuma Lake, Hawkes Bay. The 2 methods gave similar estimates of the number of bitterns using Hatuma Lake (2011: 8-9 bitterns by triangulation and 9-10 bitterns by close approaches; 2012: 8-9 bitterns using triangulation and 7-8 bitterns by close approaches). We recommend using close approaches because there appeared to be less count-error or a combination of the 2 methods to estimate numbers of Australasian bitterns on small wetlands (< 250 ha). However, neither method appears suitable for larger wetlands if the calls of distant bitterns overlap those of close bitterns, if observers get saturated by too many calling birds, or if there is insufficient time between calls to estimate distance of calls reliably and reduce the ability to distinguish individuals consistently.